Sagger



oct. 4, 1949. WENTWORTH 2,483,794 y SAGGER Filed May l0, 1945 [VEA/TOR. l ,9A/vm fe Mmm/oem Patented Oct. 4, 1949 UNITED @TENT oFFIcE ChandlerV Wentworth,- `Princeton, N. 'J., assignor to Radio Corporation of America', a corporation e of: Delaware pplication'May `10, 1945, Serial No. 593,071

4 claims. (c1. 10e- 57) dielectricelements composed in part of one'of K the titanium compounds. Such elements are used in electric condensers.

-It-'hasl beenproposedft'o make dielectric ele-1l ments'for condensers oi'mixtures containlng cer'4 taintitanate compounds such as barium strontium titanate. Withv one method of manufacture heretofore' proposed, preformed condenser elements'containing the compoun'd'areburiedv inelectricaily fused zirconium oxide sand and then heated in a furnace. After removal; thesurfaces due to reaction are pitted and cracked, requiringa grinding operation to enable use of the elements in condensers.

Or, the mixture maybe pressed into proper shapes (such as in the form of discs) and placed on a saggar made of such material as zirconium silicate compounds. The saggar Y carrying `the condenser elements may then be placed within` avmuie and red in a suitable kiln, such as an electric furnace. During the heating process, I have found that the zirconium silicate saggar, as well as the munie, react with the elements being heated, causing pitting and cracking of the surfaces.

Here again, in order to make the elements useful for condensers, the surfaces must be ground olf. rihis, of course, involves a wastev of material and increased costs, due to the labor involved `in the grinding and trimming processes. Furthermore, this difficulty limits the thinness of theV condenser elements which can be produced.

Moreover, I have found that even after grinding, a certain amount of impurities remain in the condenser element, as a result of which there' is impairment, causing departures from desired dielectric constants, power factors and dielectric strengths.

My present invention overcomes the foregoing difculties and disadvantages. In accordance with my invention, an improved saggar is provided which does not react with the elements which it supports during heating.

Briefly, in accordance with my invention' I iirst prepare a saggar (which may be termed a4 rough saggar) which contains approximately to 20% of the compound, such as a titanate used in the condenser dielectric element that is to be fired upon the saggar. This saggar is heated within a mule upon a zirconium silicate plate.

Aricertain amount ofreaction due to the heating follows,` resulting in afvvorking or rough sag-gar. The pitted surfaces 'of the rough saggar'are th'en ground on?. A second saggar is then madesofacomposition similar to that of the rough sa'ggarjv prior toheating. The'secon'd saggar, however, is carried: by the rough-saggar wherrthei'former-r is heated in the furnace.A It will be 'fndthfat when 'removed from the furnace, the second :saggar, carried by theV nist,- is suhstantially free of j pitting Aand free "offimpurities, This saggaris then used'to carrythe-desired condenser "ele-v mentsduring the heating process. When thisf isvdone, Vii'fvvill be found that substantially -no re-V action takes placev resulting inYelements-win are free of pitting and which have substan predetermined and desired-'dielectric cc'irfi'stants,

voltage breakdown strengths and power'iactorsf lit-should be noted thatfthe remainingv %*`o'fy the saggar is made of a highly refractoryfmal terial such"V as electrically fused zirconium oxidef In passing, it istobe noted that if it Wereatl'-w tempted to make a rough-saggar 'out of the''material or mixture forming the dielectricA elements, sucha saggar'would fuse to thev dielectric'elements which one might seek to nre thereon.

The invention will be more clearly-understood' by referringfto the accompanying drawings in which: 'i 5 vFig. l isvaJ perspective View of a rough'saggar.

Fig. 2 is a perspective view of av good or smooth' saggar made in accordance with this invention.`

Fig. 3 isa perspective View of a modified form of saggar made in accordance with the present invention. y

vliiggis a perspective View of condenser elements supported on the saggar of Fig. 2.

Figf'5 is a perspective views of condenser' elements supported on the saggar of Fig. 3.

Fig.'6is1aperspectiveview of saggar and condenser elements ready to be nred.

Fig. 'T is a perspective View of a condenser using a'dielectric element made by use of the improved saggar of this invention.

Referring now in detail tothe accompanying drawings and considering a more specific ex amples suppose it is desired t0 make a ceramic condenser body or dielectric element of. barium. strontiumY titanate. A saggar for such a'bocly is made in accordance with my present invention..

in the following way. Amixture for' the roughl saggar i Aas -shownfin Fig. 1 is first prepared. This. mixture consists of, for example, 15% of .barium strontium -titanate (Ba Sr(TiO3)2) and 85%.*off` electricallyf'fused --zircon-iumv oXide- ZrOaffbyweight. This mixture is then used to form 92% of a wet mixture, the remaining 8% of the latter being a 7% methyl cellulose solution. This solution, as is known, consists of 93% water by weight and 7% of methyl cellulose. The methyl cellulose used in the wet mixture completely disappears during the heating within the oven. It serves as a temporary reactionless binder.

The 8% value referred to above is not a fixed one and may vary from to 10%, depending upon the different types of molding used for forming the saggar. In the event that the amount of solution used varies from 5% to 10%. then, of course, the amount of the mixture of barium strontium titanate and electrically fused fused zirconium oxide will vary from 95% to 90% respectively.

The wet mixture is then formed to any desired shape in a press. For example, the shape may be as shown in Fig. l, simply a rectangular block. This is baked to 100 F. higher than the temperature at which it is expected to use the saggar eventually. This baking temperature may run from 2,300 to 3,000 F. The heating process may require four hours to bring the saggar up to temperature, four hours at the temperature, and then-the saggar is allowed to cure or cool with the furnace.

The saggar previously referred to, may, for the firing operation, be carried on a zirconium silicate plate. When removedfrom the furnace, it will be found to be pitted and rough (as indicated by lines 2 of Fig. 1) due to reaction within the furnace during heating. The majority of the rough spots are ground off so that the first saggar may now be termed the rough saggar. A second saggar shown by Fig. 2 is made up of asimilar mixture or composition, and is then baked in a similar way. However, this second saggar is supported by the previously prepared rough saggar during the baking process. Thus, the second saggar is made of an 8% mixtureof a '7% methyl cellulose solution, and 92% by weight of a mixture containing by weight of barium strontium titanate, and 85% by weight of zirconium oxide. The ceramic condenser elements are to be carried by the second saggar. The second saggar is baked as before stated, 100 F. higher than the temperature at whichit is expected to be used for firing. After heating up with the oven to the proper temperature in four hours baking at the proper temperature for four hours, and cooling the saggar with the furnace (rapid cooling would cause cracking of the saggar), the second saggar is removed. It will be found, as shown in Fig. 2, that the second saggar is free of pitting. This saggar may -have a fiat top as shown in Fig. 2, or it may be shaped with a plurality of ridges 4 as shown in Fig. 3, so that its upper surface carries a plurality of longitudinal strips parallelly arranged.

The saggars of Figs. 2 and 3 may now be used for the firing of the condenser element. In this case, the saggars of Figs. 2 and 3 containing the compositions indicated, and made as described, will be very useful for firing dielectric condenser elements E made of barium strontium titanate.

As shown in Fig. 4, the latter may be shaped inl pared saggar of Fig. 2 upon the rough saggar of Fig. 1. These two saggars in turn may be carried by a zirconium silicate saggar 20, and all saggars together with condenser elements 6 of barium strontium titanate may then be placed in a muiile 26 before ring in an electric furnace (not shown). The assembly of Fig. 6 is placed in the furnace and heated no higher than F. less than the temperature to which saggar 3 was subjected.

The discs 6 referred to hereinabove may be made very thin, if fired upon improved saggars made in accordance with my present invention. Thin discs are especially desirable in electrical condensers. If it is attempted to fire thin discs of barium strontium titanate according to methods heretofore employed, it will be found to be impossible to do so because of reactions customarily encountered which will cause pitting and perforation of the thin discs of barium strontium titanate. I have been successful in ring thin discs of this material by use of my improved saggars, which I have found to be substantially inert and free of reaction with the condenser elements. More specifically, I have successfully made condenser discs containing barium strontium titanate and have baked them to a temperature of 2,400 F., without pitting or deterioration, upon saggars made in accordance with Fig. 2 herein. I have made condenser discs as thin as 100th of an inch and having diameters varying from 1A; t0 3 inches.

Discs, containing barium strontium titanate or similar material, made in accordance with my present invention may be used in condensers of a form such as shown in Fig. 7.

Thus, after baking the barium strontium titanate buttons 6, they are provided with silver electrodes 'I and 8. The silver may be placed on opposite sides of each button by rst placing the button in a plastic stencil (not shown) and burnished silver paste applied, The paste is squeezed through a silk screen (not shown) on the electrode surfaces. The condenser elements 6 with the silvered surfaces are then rebaked to about 1,380 F., then allowed to cool. Copper electrode terminals 9 and l0 are then soldered to the` silver electrode surfaces in the following way.

The copper electrode terminals are tinned with a solder containing 48% lead, 48% tin, and 4% silver. An excess of solder is left on the electrode terminals. The hot button is taken from the drying oven and placed between the two copper electrode terminals in a jig. The copper electrodes are heated with a small ame until the solder melts and wets the silver on the button. VAs soon as the slder has solidied, the condenser unit is dipped in a polyvinyl acetal enamel and placed in an oven at 250 F. for fifteen minutes, when another coat of a polyvinyl acetal Vis appliedto keep out moisture. After fteen minutes at 250 F., the unit is placed in a mold and a casing of polystyrene (or a methyl methacrylate casing Il) is molded on and allowed to cool to room temperature. The capacity of the condenser so made will stabilize within twenty-four hours.

The terminals 9 and I0 protrude beyond the casing for connection purposes. In all operations, cleanliness is important. Small percentages of impurities will raise the power factor as much as ten times. Contact with the surface of the red buttons should be avoided until the silver has been applied. The buttons should not be allowed to reach room temperature until they have been coated and molded. If they do reach hour before they are coated. The capacity of a,

unit made up by the above process had an electric capacity of approximately r750 micro-microfarads.

In general, referring again to Figs. 2 and 3, the saggar of my invention should contain apn proximately to 20% of the titanate that is to be fired on the ceramic. For example, suppose the ceramic body to be fired is a lead titane ate. Then the saggar should contain 15% to by Weight of lead titanate and the re mainder should be electrically fused zirconium oxide. Ordinary zirconium oxide may be einployed, but I prefer to use the electrically fused type in View of its superior thermal properties.

Other titanates may be employed, such as magnesium titanate and calcium titanate, in the mixtures for the dielectric elements and saggars.

The invention, moreover, is not limited to saggars employed for the manufacture of dielectrics, but may also be used in the manufacture of many other devices.

Having thus described my invention, what I claim is:

1. A saggar made of a mixture containing 15 to 20% by Weight of a titanate from the class consisting of barium, strontium, calcium, magnesium, and lead titanates, and to 80% by Weight of zirconium oxide.

2. A saggar made of a mixture containing 15% to 20% by Weight of a titanate from the class consisting of barium, strontium, calcium, magnesium, and lead titanates, and 85% to 80% by weight of fused zirconium oxide.

3. A saggar containing 15% of barium strontium titanate and 85% by Weight of zirconium oxide.

4. A saggar made from a wet mixture of 15% by weight of a titanate from the class consisting of barium, strontium, calcium, magnesium, and lead titanates, and 85% by Weight of zirconium oxide.

CHANDLER WENTWORTH.

REFERENCES CITED The following references are of record in the ie of this patent:

UNITED STATES PATENTS Number Name Date 1,448,011 Tone Mar. 13, 1923 2,143,003Y Gould Jan. 10, 1939 2,220,411 Kinzie Nov. 5, 1940 2,402,517 Wainer June 18, 1946 OTHER REFERENCES Searle Refractory Materials (1924), page 447. 

